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Este estudio se ha realizado para evaluar el origen de las rocas carbonatadas del Eoceno Inferior- Oligoceno Superior de la formacion Gaziantep en las proximidades de Egil-Diyarbakir, ubicada en Anatolia Oriental, Turquia. Para lograrlo, se investigaron las propiedades estratigraficas y petrograficas, asi como las composiciones quimicas e isotopicas de las 56 muestras representativas de rocas carbonatadas tomadas de cuatro secciones estratigraficas medidas dentro de la formacion Gaziantep. Segun los datos obtenidos, la mayoria de las rocas carbonatadas examinadas se han clasificado como dolomita (D), dolomita calcitica (CD), caliza dolomitica (DL) y caliza (L). Las muestras tienen valores de Ce que van de 0,23 a 1,55, con un promedio de 0,80, lo que indica un entorno de formacion oxica. Los valores de Eu varian de 0,00 a 1,89, con un promedio de 0,97. Los valores de Ce de las muestras presentan una anomalia negativa, mientras que los valores de Eu muestran una anomalia parcialmente positiva. Por lo tanto, los valores de REE de las rocas carbonatadas sugieren que la contaminacion terrestre es inexistente o insignificante. Los valores de [delta]18O para las calizas investigadas varian entre -7,83 [por millar] y -5,33 [por millar] PDB, mientras que los valores de [delta]13C varian entre -0,17 [por millar] y [por millar] - 6,90 [por millar] PDB. Las dolomias, por otro lado, tienen valores de [delta]18O que varian entre -7,74 [por millar] y -1,23 [por millar] PDB y valores de [delta]13C que varian entre -6,79 [por millar] y -0,68 [por millar] PDB. Ademas, la temperatura de formacion de las dolomias varia entre 39oC y 86oC. Los datos obtenidos, incluyendo la temperatura y las composiciones de [delta]18O y [delta]13C, mostraron evidencia significativa de que la dolomita se formo en la Formacion Gaziantep en dos etapas distintas. La primera etapa implico la formacion de dolomita (D) en la zona de mezcla a bajas temperaturas durante la etapa diagenetica temprana. La segunda etapa implico la formacion de dolomita (CD y DL) en aguas meteoricas y temperaturas en aumento durante la etapa diagenetica tardia. Keywords Dolomita * Formacion Gaziantep * Diagenesis tardia * Isotopo estable * Elementos de tierras raras

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO₃)₂] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the “dolomite problem.” Accordingly, the genesis of these dolostones is usually attributed to burial–hydrothermal dolomitization of primary limestones (CaCO₃) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (TΔ47) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our TΔ47 record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our TΔ47 record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δ18O values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.

Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative δ13Ccarb excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform δ13Ccarb compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large δ13Ccarb excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.

Iron is essential for the proper functioning of the entire production chain of marine biomass and of the ocean’s food web. However, its biogeochemical behavior often makes it a limiting factor in ocean functioning. On a geological timescale, the initially reactive-iron reservoir is most often stored as iron sulfides in the sedimentary record. This study focuses on episodes occurring during the earliest stages of diagenesis. It shows that there is a cycle of reversible transformations of iron states before the situation becomes fixed by the formation of iron sulfides, the most emblematic of which is pyrite. The material studied here is an alternation of diagenetic limestone beds and marly interbeds of Tithonian age, observed along the cliffs of the Boulonnais region (Strait of Dover, France), and known as the Assise de Croï Formation. The early, authigenic carbonates of the limestone beds trapped iron-bearing, authigenic minerals, notably magnetite. This made visible the iron cycle, which can be described as cryptic because it goes unnoticed if nothing reveals it. This “fossilization” of the early stages of iron distribution, through the precipitation of diagenetic limestone, allows for a more refined understanding of the carbon cycle, particularly in its shallow marine compartment: indeed, the cryptic iron cycle actively participates in the remineralization of fragile (labile) organic matter.

The marine Upper Jurassic rocks of the Franconian Alb consist largely of micritic carbonate of partly dolomitized reef mounds and bedded basinal limestone. All carbonates were lithified in the shallow (centimeters, meters) subsurface and have a wide range of ∂ 13 C (≤ + 3‰ to − 10‰VPDB) but always negative ∂ 18 O (− 1 to − 6‰VPDB). Dolomite and reef limestone show the highest ∂ 18 O and ∂ 13 C values. The most negative ∂ 13 C (≥ − 10‰) occurs mainly as cement in dolomite of a basinal, partly dolomitic, biostrome interval. Basinal limestone shows intermediate ∂ 13 C values. Because freshwater diagenesis and elevated temperatures cannot explain the observed isotope values, pH is here considered a major factor influencing the isotope signal of micritic limestone. The bulk sediment isotope signal was reset to lower values, from an original lime mud with ∂ 13 C ≥ 3‰ and a ∂ 18 O of ≥  + 1‰, as a result of biochemically induced diagenesis. Carbonate, probably mostly aragonite but occasionally including dolomite, was dissolved in a zone where low pH developed as a result of organic matter degradation. Dissolved carbonate was translocated by diffusion and re-precipitated as cement (ca. 50vol%) in a zone with elevated pH where all in situ lime mud ∂ 18 O was reset. Imported cement carbonate precipitated in equilibrium with the pore fluid with negative isotope values, whereas ∂ 13 C of the in situ lime mud remained unmodified. The negative shift of the bulk ∂ 13 C and ∂ 18 O is variable and depends on pH and the contribution of 12 C from anaerobic sulfate reduction in the zone of cement precipitation. This produced an ubiquitous covariance of ∂ 18 O and ∂ 13 C. Incorporation of seawater-derived Mg 2+ during recrystallization of carbonate can account for the local dolomitization. Elevated 87 Sr/ 86 Sr ratios are explained as a result of interaction of clay minerals with the stationary pore fluids. This study shows that the isotopic signal produced by biochemically induced shallow submarine subsurface carbonate diagenesis can be indistinguishable from freshwater diagenesis, that ∂ 18 O and ∂ 13 C of the bulk rock are always reset, and that carbonates can show, in the presence of clay minerals, elevated 87 Sr/ 86 Sr ratios even when the pore fluids were never exchanged. Graphical Abstract

The 18O/16O ratio of cherts (δ18Ochert) increases nearly monotonically by ~15‰ from the Archean to present. Two end-member explanations have emerged: cooling seawater temperature (TSW) and increasing seawater δ18O (δ18Osw). Yet despite decades of work, there is no consensus, leading some to view the δ18Ochert record as pervasively altered. Here, we demonstrate that cherts are a robust archive of diagenetic temperatures, despite metamorphism and exposure to meteoric fluids, and show that the timing and temperature of quartz precipitation and thus δ18Ochert are determined by the kinetics of silica diagenesis. A diagenetic model shows that δ18Ochert is influenced by heat flow through the sediment column. Heat flow has decreased over time as planetary heat is dissipated, and reasonable Archean-modern heat flow changes account for ~5‰ of the increase in δ18Ochert, obviating the need for extreme TSW or δ18Osw reconstructions. The seawater oxygen isotope budget is also influenced by solid Earth cooling, with a recent reconstruction placing Archean δ18OSW 5 to 10‰ lower than today. Together, this provides an internally consistent view of the δ18Ochert record as driven by solid Earth cooling over billion-year timescales that is compatible with Precambrian glaciations and biological constraints and satisfyingly accounts for the monotonic nature of the δ18Ochert trend.

Vivianite, Fe3(PO4)2 · 8 H2O, is a ferrous iron phosphate mineral which forms in waterlogged soils and sediments. The phosphorus (P) bound in its crystal lattice is considered to be immobilised because vivianite is stable under anoxic, reducing, sedimentary conditions. Thus, vivianite formation can make a major contribution to P retention during early diagenesis. Much remains unknown about vivianite in sediments, because technical challenges have rendered direct identification and quantification difficult. To identify vivianite and assess its significance for P burial during early diagenesis we studied the consequences of a 1992/1993 in-lake application of FeCl3 and Fe(OH)3 aimed at restoring Lake Groß-Glienicke (Berlin, Germany). In a novel approach, we firstly applied a heavy-liquid separation to the iron-rich surface sediments which allowed direct identification of vivianite by X-ray diffraction in the high-density (ρ > 2.3 g cm−3) sediment fraction. Secondly, we assessed the contribution of vivianite to P retention, combining results from chemical digestion with magnetic susceptibility data derived from magnetic hysteresis measurements. Scanning electron microscopy revealed that the dark blue spherical vivianite nodules were 40–180 μm in diameter, and formed of platy- and needle-shaped crystal aggregates. Although equilibrium calculations indicated supersaturation of vivianite throughout the upper 30 cm of the sediment, the vivianite deposits were homogeneously distributed within, and restricted to, the upper 23 cm only. Thus, supersaturated pore water alone cannot serve as a reliable predictor for the in situ formation of vivianite. In Lake Groß -Glienicke, vivianite formation continues to be triggered by the artificial iron amendment more than 20 yr ago, significantly contributing to P retention in surface sediments.

Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low‐field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low‐temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo‐single‐domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

Formation and subsequent burial of authigenic phases have been recognized as a key process removing silicon from the ocean. However, the effect of authigenic phase formation on the isotopic mass balance of silicon in the ocean is not clear. Here, we constrain the apparent silicon isotope (δ30Si) fractionation associated with early diagenesis by measuring δ30Si signatures of pore fluids and authigenic phases in a South Atlantic sediment core. The δ30Si offsets between authigenic phases and pore fluids vary between −2.9 and −0.4‰, supporting a substantial negative δ30Si fractionation during early diagenesis. The variable apparent δ30Si fractionation covaries with the amount of sedimentary lithogenic materials and may be attributable to the associated kinetic effects. Overall, our data show that authigenic phases buried along with their precursor phases preferentially remove isotopically light silicon from the ocean, with implications for the isotopic mass balance of the marine silicon cycle.

The upper Turonian - lower Campanian Supersequence is characterized with fine-grained, mixed siliciclastic-carbonate middle shelf to sub-basinal depositional system. The present study included the study of vertical and lateral changes of the diagenesis processes of the Gulneri shale, Kometan and Mushorah Formations within the sequences of the (upper Turonian-lower Campanian) cycle in two subsurface sections within the Dome of Daoud in the Bai Hassan field - northern Iraq. The first is represented by a section (BH-16) with a thickness of (213) m. In contrast, the second is represented by a section (BH-86) and a thickness of (233) m. The lithostratigraphy of the rock units of this study are consisting (206) slides were studied, (80) for well (BH-16) and (126) for well (BH-86).The study showed that the sequences of this cycle are unconformity with the lower Dokan Formation and upper contact with Hartha. The petrographic study showed that the rocks of the study were exposed to many diagenesis processes, like cementation, dolomitization, neomorphism, dissolution and compaction according to the degree of their prevalence with a large spread of locally authigenic minerals represented by the widespread pyrite mineral during the two sections of the study, in addition to a few presence of glauconite mineral. Depending on the petrographic description of the diagenesis processes, the vertical and lateral changes of the diagenesis processes were studied, and the diagenesis environments of the formations under study were deduced, where it was found that all the rocks of the two sections of the study were formed in the early stages of the diagenesis processes within the vadose environments as a result of exposure to tectonic processes that helped uplift their rocks to the surface and be affected by meteoric water.